The proposal investigates the physiological importance of 3 types of stretch-activated (SA) ion channels found in the amphibian renal proximal tubule. These channels may be important for both osmoregulation and electrolyte homeostasis in this nephron segment. Upon exposure to hypotonic media many types of renal cells initially swell and then exhibit a regulatory volume decrease (RVD). If swelling stretches the cell membrane enough to activate SA channels, loss of cell K together with an anion and water could explain the regulatory decrease in cell volume. Since the proximal tubule does not normally encounter drastic fluctuations in osmolarity, SA channels may be more important for electrolyte homeostasis. The slight degree of cell swelling associated with apical Na-substrate cotransport could stimulate SA channels at one or both sides of the cell. The resultant increase in potassium conductance would enhance K exit from the cell down its electrochemical gradient; thereby balancing the additional K entry arising from Na-stimulation of the Na-K ATPase during increased availability of luminal substrates. An understanding of this intrinsic feedback stabilization process would be important for treating conditions where homeostatic regulation has been impaired. The proposed experiments will utilize the patch-clamp technique to study stretch-activated channels in three preparations of the amphibian proximal tubule: the isolated tubule, the open tubule, and the isolated polarized cell. The particular advantages of each of these preparations would be exploited to provide a comprehensive description of these channels in cell-attached patches, excised patches and isolated whole-cells. All of the specific aims examine the characteristics of stretch and volume activation and its relevance for osmoregulation and electrolyte homeostasis. The isolated, polarized proximal tubule cell is a particularly useful preparation since volume measurements can be performed simultaneous with either single channel recording or whole-cell recording.